The Roman Calendar

The Ptolemaic System
The Ptolemaic System
From Greek Astronomy by Sir Thomas L. Heath.

Our modern calendar is closely based on that implemented by Julius Caesar during 46-45BC, and amended by Pope Gregory XIII in 1582AD. The ancient Roman calendar was closely linked to the science of astrology and the teachings of Claudius Ptolemaeus, which were prevalent throughout the entire lifetime of Imperial Rome. Ptolemy's teachings were based, in turn, on those of Plato and Pythagoras who both expounded a geocentric, 'earth-centred' view of the universe in which the sun, moon and planets all revolved about a stationary Earth, positioned as it should be, at the very hub of the cosmos.

Among the lessons published in Ptolemy's astronomical thesis Syntaxis were; "The earth does not change its position in any way whatever", also "Arguments against the earth's rotation". These theories are now known to be infactual but they were not refuted until ad1543 when the Polish astronomer Mikolaj Kopernik (a.k.a. Nicolaus Copernicus 1473-1543) published his almost-heretical work which expounded a heliocentric or sun-centred 'solar-system'.

The Division of the Day

Like us, the Romans divided each day into 24 hours, and they assigned 12 to the daytime and 12 to the night. These did not run from midnight to midnight as our modern method of timekeeping does, but from sunrise to sunrise. This effectively means that the length of the Roman hour varied according to the season, so that during the summer solstice¹ around June 21st when the period of daylight is considerably longer than the night, the twelve hours assigned to the daytime would each have to be 1 hour and 16 minutes long, while conversely, during the short days of the winter solstice around December 21st, each daylight hour would be only 44 minutes long.

There were only two days during the entire year when the Roman day contained hours of exactly 60 minutes. These dates occurred during the equinoxes,² when the length of the day is exactly equal to that of the night; the vernal equinox occurred every year around March 21st, and the autumnal equinox about September 21st.

This fluid method of timekeeping was perfectly natural to your average Roman, who was not governed by the same rigid schedules prevalent in our modern technological society and carried neither a wristwatch nor an iPad.

  1. From Latin solstitium, from sol 'sun' + sistere 'to stand still'.
  2. From late Latin equinoxium, early Latin aequinoctium, from aequi 'equal' + nox 'night'.

Table of Daylight Hours at the Solstices

Winter Solstice
I.prima7:338:17 a.m.
II.secunda8:179:02 a.m.
III.tertia9:029:46 a.m.
IV.quarta9:4610:31 a.m.
V.quinta10:3111:15 a.m.
VI.sexta11:1512:00 noon
VII.septima12:0012:44 p.m.
VIII.octava12:441:29 p.m.
IX.nona1:292:13 p.m.
X.decima2:132:58 p.m.
XI.undecima2:583:42 p.m.
XII.duodecima3:424:27 p.m.
Summer Solstice
I.prima4:275:42 a.m.
II.secunda5:426:58 a.m.
III.tertia6:588:13 a.m.
IV.quarta8:139:29 a.m.
V.quinta9:2910:44 a.m.
VI.sexta10:4412:00 noon
VII.septima12:001:15 p.m.
VIII.octava1:152:31 p.m.
IX.nona2:313:46 p.m.
X.decima3:465:02 p.m.
XI.undecima5:026:17 p.m.
XII.duodecima6:177:33 p.m.
Table adapted from Daily Life in Ancient Rome by Jerome Carcopino pp.167-8.

It should be noted that the times of rising and setting of the Sun also varies with geographical latitude, and the data in the above table shows the length of the daylight hours at the latitude of Rome itself; this table would not be valid for many other cities in the Roman world.

The Days of the Week

Macrobius tells us that at first, the Romans used the ancient Etruscan Market Week, which consisted of seven working days followed by a market day called the Nunindae. During this eighth day many public auctions were held, and Varro joked that the rural population shaved and came into the city, thus the Nunindae became a day of festivity.

"The custom, however, of referring the days to the seven stars called planets was instituted by the Egyptians, but is now found among all mankind, though its adoption has been comparatively recent; at any rate the ancient Greeks never understood it, so far as I am aware." (Cassius Dio, History of Rome, XXXVII, 18.1)

The astrological or planetary week of seven days is thought to have started in Persian theology, and by the end of the first century AD was in common usage throughout the whole Mediterranean world. Although the planetary week was recognised by the emperor Augustus, he continued to run the ancient market calendar alongside, and it was not until 321AD during the rule of emperor Constantine that the astrological week became fully established in Roman law.

The Astrological Week

Roman DayTranslationModern
Modern Derivation
dies Saturni'the day of Saturn'SaturdayDirectly from Latin.
dies Solis'Sun day'SundayLikewise.
dies Lunae'Moon day'MondayDitto.
dies Martis'the day of Mars'TuesdayOE Tiwesdaeg 'The day of Tiw',
from Norse Tysdagr.
dies Mercuris'the day of Mercury'WednesdayOE Wodnesdaeg 'the day of Woden',
from Norse Odinsdagr
dies Iovis'the day of Jupiter'ThursdayOE Thursdaeg 'the day of Thor',
from Norse Thorsdagr
dies Veneris'the day of Venus'FridayOE Frigesdaeg 'the day of Freya',
from Norse Freyjasdagr
  1. Compare Modern Dutch: Woensdag.
  2. Compare Old German: Donares Tag 'The Thunderer's Day'; Modern German Donnerstag.
  3. Compare Old High German Friatag, and Old Frisian Friadei.

A short digression into Norse mythology

The last four days of the week are named in English after Viking gods, and it behooves our purpose to here present a short summation of the four gods from the Norse pantheon after which our modern day names have been derived.

Or Tiu, was the name of the Anglo-Saxon god of war and the sky. A.k.a. Norse Tyr (son of Odin) or Germanic Tiwaz. This defender-god was also a sage, and was likened by the Romans with Mars, their own god of war. He was an ancestor god of Norse mythology, tradtionally depicted as an old sage dressed in animal skins and bearing a sceptre in his right hand.
Alternately spelled Othinn, this god was also known as Woden in Britain and Wotan or Wodan in Germany. He was god of magic, poetry, ecstacy, wealth and the gain of riches, possibly with healing, and also the dead. He was leader of the Aesir, the Norse pantheon, and the Valkyries were his to command; these warrior-maidens conducted the souls of dead heroes to the Halls of Valhalla, over which he held dominion. His most fanatic followers were the Berserkers, who fought 'bare-sark', without armour or clothing, believing that their swift death in battle would be rewarded by a place in Valhalla. He appeared as a tall, spear carrying warrior accompanied by a raven, an eagle and a wolf, and riding his eight-legged horse Sleipnir, a magical beast capable of carrying him across the sky. The Romans equated him with Mercury. Important myths associated with Odin were the 'regaining of the mead of inspiration', 'the sacrifice of an eye to gain knowledge', and 'the aquisition of runic lore through hanging in torment upon Yggdrasil, the World-Tree, the Guardian Ash of the Aesir. Odin was doomed to be devoured by Fenris Wulf during the battle of Ragnarok, which name in Iclandic means 'the twilight of the powers', and signified the end of the Aesir, as both Thor and the treacherous Loki were also killed.
His name stems from the Old Norse word for thunder, thorr, equivalent to the German god Donar, Anglo-Saxon Thunor also Taranis. Equivalent with Roman Jupiter¹ or Greek Zeus, a sky-god who controlled the winds and weather, and made thunder sound by hitting the earth with his mighty hammer, which was manifested on earth as the destruction caused by bolts of lightning. He was also attributed with upholding the law of the Aesir, presided over the Law Assembly, and the protection of the community was in his charge. Associated with the oak. Oaths were sworn on his sacred ring. Depicted as a red-bearded waggon driver with a ravenous appetite, who caused thunder to sound as his cart lumbered across the sky. His symbols were the hammer and the swastika. Tales include his encounter with the frost giants and fishing for the world-serpent. He perished in battle with the serpent during Ragnarok, the final battle of the Aesir.
One of the Vanir, the Scandinavian fertility deities, linked with spirits of the land and with dead ancestors. She was the goddess of love and fecundity, and the sister of Freyr, the god of male fertility. Their father was Njord, patron god of ships, and god of all the seas and lakes. Her image was loaded on a wagon and carted from farmhouse to steading, to ensure a fruitful season's harvest. She has been identified with Odin's wife, Frigg, also with the Germanic goddess Frija, her Roman equivalent was Venus and in Greek mythology, Aphrodite. Her associated symbols were a ship and a golden boar.

Why the Names of the Days Do Not Follow Pythagoras

Like the rest of the ancient Mediterranean world, the Romans believed that each day of the week was ruled by a specific god, in particular, the seven gods after whom the sun, moon and planets were named. These celestial-bodies were known to lie at varying distances from the Earth, and listed in order of descending remoteness they were; Saturn, Jupiter, Mars, the Sun, Venus, Mercury and finally, the Moon. If this is the case, why then, are the Roman days of the week not listed in this order? The answer lies in the fact that these seven Roman deities ruled not only over each day of the week, but also over each hour of the day; The Roman historian Cassius Dio explains:

"1 If you begin at the first hour to count the hours of the day and of the night, assigning the first to Kronos¹ [Saturn], the next to the great god [Zeus/Jupiter], the third to Ares [Mars], the fourth to Helios [the Sun], the fifth to Aphrodite [Venus], the sixth to Hermes [Mercury], and the seventh to Selene [the Moon], 2 according to the order of cycles which the Egyptians observe,² and if you repeat the process, covering thus the whole twenty-four hours, you will find that the first hour of the following day comes to the Sun. 3 And if you carry on the operation throughout the next twenty-four hours in the same manner as with the others, you will dedicate the first hour of the third day to the Moon, and if you proceed similarly throughout the rest, each day will receive its appropriate god. This, then, is the tradition." (Cassius Dio, History of Rome, XXXVII, 19.1-3)
  1. Dio was a Greek, and therefore named the celestial bodies according to his native tradition.
  2. The Ptolemaic system.

The Calends, the Ides and the Nones

Every Roman month contained three days of particular importance;

All other dates were calculated by counting backwards inclusively from these established days, thus March 22nd is '10 days before the Calends of May', October 10th becomes '6 days before the Ides of October' and January 2nd, '4 days before the Nones of January'.

The Julian Calendar

It is known that the Babylonians utilized a calendar of 12 months each containing 30 days, thus they had a year of 360 days. The Romans, prior to the reformation of Caesar, used a system of months of irregular length giving a 355 day civil year, into which an extra month of 22 or 23 days was intercalated every other year. On the 1st January 45BC, after tacking 90 days onto the end of 46BC in order to bring the seasons back into line, Caesar introduced his Julian Calendar, which did not so much reform the ancient Roman calendar but abandon it, instituting instead the familiar solar calendar of 365¼ days. The ten extra days required to bring the Roman year into line with the solar year were divided up and added to the end of several separate months so as not to interfere too much with the existing festival schedule. In addition, an extra day was intercalated every four years in February. Cassius Dio again elucidates:

"1 ... he (Caesar) also established in their present fashion the days of the year, which had got somewhat out of order, since they still at that time measured their months by the moon's revolutions: he did this by adding sixty-seven days, the number necessary to bring the year out even. 2 Some, indeed, have declared that even more were intercalated, but the truth is as I have stated it.¹ He got this improvement from his stay in Alexandria, save in so far as the people there reckon their months as of thirty days each, and afterwards add the five days to the year as a whole, whereas Caesar distributed among seven months these five along with two other days that he took away from one month.² 3 The one day, however, which results from the fourths he introduced into every fourth year, so as to make the annual seasons no longer differ at all except in the slightest degree; at any rate in fourteen hundred and sixty-one years there is need of only one intercalary day.³" (Cassius Dio, History of Rome, XLIII, 26.1-3)
  1. Modern chronologists have determined that Caesar may have intercalated 90 days onto the end of 46BC.
  2. February.
  3. The tropical year is 365 days 5 hours and 48¾ minutes long, which amounts to an error of one day in every 128 years.

Caesar's intercalary day was not inserted at the end of the month as happens in modern times, but was placed instead between the 24th and 25th of February. Following Caesar's death the pontifices erroneously performed the intercalation every three years, and this mistake had to be rectified by another slight reform implemented by his nephew and successor Augustus Caesar.

The Roman Months of the Year

 Number of Days in Month
Mensis (Month)Origin of NameAnte CaesaremPost Caesarem
Januarius (January)God Janus2931
Februarius (February)Februa festivals ¹2828
Martius (March)God Mars3131
Aprilis (April)Aprilis ²2930
Maius (May)Goddess Maia3131
Junius (June)Goddess Juno2930
Julius (July)Julius Caesar ³3131
Augustus (August)Augustus Caesar2931
September'The Seventh Month'2930
October'The Eighth Month'3131
November'The Ninth Month'2930
December'The Tenth Month'2931
  1. Marking the end of the ancient Roman year; which thus began with the month of March/Mars.
  2. A goddess of the Etruscans, an ancient people of central Italy.
  3. July was originally named Quintillis 'the Fifth Month', and August Sextilis 'the Sixth Month'.

Thwarted Attempts by Warped Emperor's to Rename Months

Caligula renamed September 'Germanicus' after his father in 37AD, but this was overturned following his assassination and the subsequent condemnation of his memory by the senate in 42AD. September was once more renamed 'Germanicus' in 89AD, this time by the Emperor Domitian following his triumph over the Germanic Chatti tribe; he also renamed October 'Domitianus' as this was the month in which he was born. Domitian was also assassinated, his name condemned, and his acts overturned in 96AD. Commodus in 190AD renamed every month in the calendar after himself (from January): 'Amazonius' , 'Invictus', 'Felix', 'Pius', 'Lucius', 'Aelius', 'Aurelius', 'Commodus', 'Augustus', 'Hercules', 'Romanus', 'Exsuperatorius'. The acts of this megalomaniac emperor were overturned by the senate after his assassination in 192AD.

The Naming of Each Year

The post of Consule Ordinaris was an enormously important political office endowed with the greatest legislative, judicial and military authority in the Roman Republic. The office was dual, elective, and of one-year's duration, all aspirants for the position must first have served as praetor, and the two candidates who polled the most votes took up office on the first of January.

There were two divisions in the office of consul; the consules ordinarii, who were the two men who polled the most votes in the yearly consular elections, and the consules suffecti who would sometimes be appointed in republican times to replace a consul who had been killed in battle, or otherwise relieved of his office; often the next-highest polling consular candidate. The duties of 'ordinary' and 'suffect' consul were exactly the same - i.e. to ensure the smooth-running of the Roman state - but there was one important difference; the Roman year was named after the Consules Ordinarii. As one can imagine, the office of 'Ordinary Consul' was extremely sought-after, as its aquisition would forever immortalise the recipient's name in the annals of Rome.

The full list of Roman consuls is available to modern historians, dating from the foundation of the Roman republic in 509 BC, until the division of the empire in 337AD, thanks mainly to classical historians such as Varro and Cassius Dio, who each gave lists of consulars in their works, and to the diligent works of modern historians such as A. Degrassi and T.R.S. Broughton.

For a partial list of consulars (34 B.C. to A.D. 100), see our Roman Consuls Page.

Ab Urbe Condita - From the Founding of the City

Unlike our modern Christian calendar which enumerates the years from the birth of Jesus Christ, the Roman calendar counted the years ab urbe condita or "from the founding of the city". The city of Rome was founded in 753 BC, so this would bracket our list of consuls between a.u.c. 245 and 1090.

"Claudius was born ... on the Kalends of August in the consulship of Iullus Antonius and Fabius Africanus, ..." (Suetonius Claudius II.i)

The above extract from Suetonius' Life of Claudius tells us not only the day Claudius was born - the first of August - but we can also find his year of birth by reference to the list of known consulars, where we find that Fabius Africanus Maximus (the son of Quintus) and Iullus Antonius (the son of Marcus Antonius) were consules ordinarii a.u.c.744; that's 10BC to us mortal folk!

The Gregorian Calendar

The Julian calendar remained unchanged until the time of pope Gregory the Thirteenth, who in 1582 introduced an emendation which stated that the intercalation should be performed at the turn of each century only if the century was exactly divisible by 400. This amendment was required because the tropical year, as explained above (vide supra), is not exactly 365¼ days long.

When the Gregorian Calendar was first implemented it was to cause uproar throughout the Roman-Catholic world because it required the deduction of thirteen days in order to bring the calendar back into line with the seasons, and many uneducated people rioted in the streets thinking that these days had somehow been deducted from their lifespans.

The Gregorian calendar is now used throughout almost the entire modern world, the most famous exceptions being the Greek and Russian Orthodox Churches, who still observe the old Julian calendar.

For those like me, who need to cheat in Latin exams

Togodumnus' Latin/English Date JavaScript

is freely available!

Roman Appendix the correct use of BC/AD

On this page - indeed throughout the whole of Roman-Britain - the acronyms BC (Before Christ) and AD (Anno Domini) are correctly used to denote absolute dates just as we use other units such as "10m" for (10 meters) or "230v".

The controversy derives from the (false) view that "BC" is somehow offensive to non-Christians. But "Christ" is a just a Greek name meaning "anointed" or "oiled" and anointing or smearing in oil was commonly used in ancient times. It was used for medical purposes and even anointing guests with oil was a mark of hospitality and token of honour is recorded in Egypt, Greece, and Rome, as well as in Judaism.

Yes Christ derives in some way from religion, but so do many others including my own name: Michael- From the Hebrew name מִיכָאֵל "Mikha'el" meaning "who is like God?" (similarly "Emmanu-el" and other names ending "-el"). Many many names derive from the name of gods both native and foreign. (Even the name "Jesus" seems to derive from Zeus).

Christ means "anointed" in the same way that the Queen was anointed, the pope was anointed, but also in the same way most people "an-oint" themselves with sun-cream or medical ointments. This Greek name should only cause offence to Hebrew speaking Jews who want to take offence. In contrast, it quite unethical not to use /ADBC, because in all other areas of work, we try to respect the work of first discoverers by using their terminology. So why, when it was only through the work of Christian historians that we have the modern dating system, do some go out of their way to insult them? Like all research, we should try to respect those who discovered or created ideas by where possible using their terminology.

But worse still! BCE/CE is itself not PC!

In poor printing/lighting B & 8 & 8 & B are very easily confused meaning that for example I mistake 13BCE for 138CE in some papers (and I have good eyesight!). Imagine how difficult it is for the less abled! Far from being "PC", it is actually discriminatory against the disabled with poor eye sight (and we all get older and eyesight deteriates).

See: Chronology of the Ancient World by E.J. Bickerman (Thames & Hudson, London, 1980);
Greek Astronomy by Sir Thomas L. Heath (Dover, New York, 1991);
Daily Life in Ancient Rome by Jerome Carcopino (Penguin, London, 1970);
Ιστορια of Cassius Dio, trans. by Earnest Cary (Loeb, 1914);
The Collins Latin Dictionary (Collins, 1957 - 1997);
The Collins English Dictionary (Collins, 1998);
Revised Latin Primer by Benjamin Hall Kennedy (Longmans, London, 1953);
Page Citation: Mike Haseler, Kevan White (2018) "Roman Britain: Roman Calendar"